The present invention relates generally to microporous sheet products and relates more particularly to a novel microporous sheet product and to methods of making and using the same.
Microporous sheet products are well-known and commonly used articles found in items as diverse as, for example, electrochemical batteries, food packaging materials, and ultrafiltration devices. For example, in electrochemical batteries, microporous sheet products are commonly used as battery separators. Typically, an electrochemical battery includes at least one pair of electrodes of opposite polarity and, in many cases, includes a series of electrode pairs of alternating polarity. The current flow between the electrodes of each pair is maintained by an electrolyte. Depending on the nature of the battery system, the electrolyte may be acidic, alkaline, or substantially neutral, and the battery may a primary or secondary (rechargeable or storage) system. For example, in alkaline storage batteries, which include, but are not limited to, primary, secondary, nickel, zinc and silver cells, the electrolyte is generally an aqueous solution of potassium hydroxide. By contrast, in lead acid batteries, the electrolyte is typically a sulfuric acid solution, and, in lithium storage batteries, the electrolyte is typically an organic solution of lithium salt, such as lithium trifluoromethyl sulfonate, lithium tetrafluoro borate, lithium hexafluorophosphate, or another lithium salt.
A battery separator is typically provided in a battery between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates since such direct contact would result in a short circuit of the battery. In general, it is highly desirable for the separator to possess one or more of the following qualities: (i) to be thin and lightweight to aid in providing a battery of high energy density and specific energy; (ii) to have a structure that inhibits dendrite formation between the electrode plates; (iii) to have the ability to enhance the uptake of the electrolytic composition over the electrode plates and, in so doing, to promote a substantially uniform distribution of the electrolytic composition over the electrode plates (an effect generally referred to as wicking); (iv) to provide the property of freely permitting electrolytic conduction; and (v) to have a dimensionally stable structure even during thermal excursions (internal or external heating). It is further highly desirable for the separator to be made in an economical and environmentally safe manner while being substantially free of defects, such as pinholes and the like.
Separators of the type that are conventionally used in battery systems are typically made of porous structures that, when placed in an electrolyte or electrolytic system, are capable of exhibiting a high degree of conductivity while being stable to the environment presented by the battery system. The separator may be a macroporous structure, such as in the case of nonwoven structures made of spun polymer and glass fibers. Alternately, the separator may be a microporous structure, such as in the case of polymeric films with or without fillers.
More specifically, one known type of separator comprises a nonwoven fibrous material, the nonwoven fibrous material typically having a high porosity, an average pore size of at least 10 microns, and low resistivity. An example of such a separator is disclosed in U.S. Pat. No. 4,279,979, inventors Benson et al., which issued Jul. 21, 1981, and which is incorporated herein by reference. In particular, in the aforementioned patent, there is disclosed a nonwoven fibrous substrate for a battery separator. The aforementioned substrate, which is said to be for an alkaline battery separator, is made of a lightweight, porous, heat bonded, synthetic organic sheet material having a basis weight of less than about 35 gsm and a thickness of less than about 200 microns. The major fibrous component is synthetic pulp comprising thermoplastic polyolefin fibers having a prefused microfibrillar structure similar to wood pulp. The minor fibrous component is a high tenacity polyamide fiber having a fiber length greater than about 6 mm. The heat bonding by partial fusion of the microfibrillar polyolefin is sufficient to impart to the sheet material a wet tensile strength of at least 400 g/in width while permitting retention of air permeability of about 100 liters per minute and more. The substrate is said to be particularly well-suited for use in nickel-zinc batteries.
Another known type of separator is disclosed in U.S. Pat. No. 4,283,442, inventors Machi et al., which issued Aug. 11, 1981, and which is incorporated herein by reference. In particular, in the aforementioned patent, there is disclosed a method of producing a dimensionally stable battery separator. The method is characterized by grafting acrylic acid and/or methacrylic acid onto a polyethylene film, treating the resulting membrane with an aqueous alkaline solution, and drying the treated membrane under application of tension.
Still another known type of separator comprises a microporous sheet product that is formed by extruding a composition that includes a polyolefin and a liquid plasticizer and, thereafter, removing the plasticizer to produce a sheet with a microporous structure. Conventionally, such plasticizers are high molecular weight oils (i.e., a carbon chain of 10 to 70 carbon atoms) that are selected based on their compatibility with the polymeric material during the initial steps of sheet formation including phase-separation and that are readily extractable during process formation. In particular, mineral oil is commonly used as a plasticizer for lithium battery separators. Extraction of the plasticizer is conventionally achieved by washing the plasticizer from the cooled initially-formed sheet using a compatible, low molecular weight liquid solvent (e.g., hexane). The voids resulting from the removal of the plasticizer provide substantially uniform porosity throughout the resultant separator sheet product.
Yet another example of a separator is disclosed in U.S. Patent Application Publication No. US 2013/0029126 A1, inventor Yen, which was published Jan. 31, 2013, and which is incorporated herein by reference. In particular, in the aforementioned publication, there is disclosed a sheet product suitable for use as a battery separator, as well as a method of forming the sheet product. The method comprises forming a mixture of a polyolefin and a fluid having a high vapor pressure, shaping the mixture into a sheet material and subjecting the sheet material to stretching/fluid vaporization at high temperature to form an intermediate material having a ratio of percent fluid to percent polymer crystallinity of between 0.15 and 1, followed by a second stretching/fluid vaporization at a lower temperature while removing a portion of the remainder of the fluid from the sheet. The resultant sheet is annealed and the remainder of fluid is removed to form a sheet product having a thickness comprising a stratified structure of small and larger pore layered configuration across its thickness.
A further example of a separator is disclosed in U.S. Pat. No. 6,461,724 B1, inventors Radovanovic et al., which issued Oct. 8, 2002, and which is incorporated herein by reference. In this patent, there is disclosed a microporous material comprising a polypropylene polymer having at least 20 percent crystallinity; and a compatible, amorphous, glassy polymer, wherein said polymers are miscible in a compound when heated above the melting temperature of the polypropylene polymer and phase separate from the compound when cooled below the crystallization temperature of the polypropylene polymer. A preferred amorphous glassy polymer compatible with polypropylene is said to be cyclic olefinic copolymers including ethylene norbornene copolymers. Compounds said to be suitable for the melt-blending of the polypropylene polymer with the amorphous, glassy polymer include mineral oil and mineral spirits.
Additional documents that may be of interest include the following, all of which are incorporated herein by reference: U.S. Pat. No. 8,748,028 B2, inventors Takita et al., issued Jun. 10, 2014; U.S. Pat. No. 8,728,617 B2, inventors Benenati et al., issued May 20, 2014; U.S. Pat. No. 8,703,283 B2, inventors Goerlitz et al., issued Apr. 22, 2014; U.S. Pat. No. 8,262,973 B2, inventors Lee et al., issued Sep. 11, 2012; U.S. Pat. No. 8,092,877 B2, inventors Jester et al., issued Jan. 10, 2012; U.S. Pat. No. 8,048,520 B2, inventors Hayes et al., issued Nov. 1, 2011; U.S. Pat. No. 7,288,316 B2, inventor Jester, issued Oct. 30, 2007; U.S. Pat. No. 6,696,524 B2, inventor Hausmann, issued Feb. 24, 2004; U.S. Pat. No. 6,242,127 B1, inventor Paik et al., issued Jun. 5, 2001; U.S. Pat. No. 6,013,151, inventors Wu et al., issued Jan. 11, 2000; U.S. Pat. No. 5,939,181, inventors Kumano et al., issued Aug. 17, 1999; U.S. Pat. No. 5,336,573, inventors Zuckerbrod et al., issued Aug. 9, 1994; U.S. Pat. No. 4,699,857, inventors Giovannoni et al., issued Oct. 13, 1987; U.S. Pat. No. 4,539,256, inventor Shipman, issued Sep. 3, 1985; U.S. Pat. No. 4,285,751, inventors Feinberg et al., issued Aug. 25, 1981; U.S. Pat. No. 4,210,709, inventors Doi et al., issued Jul. 1, 1980; U.S. Pat. No. 4,024,323, inventor Versteegh, issued May 17, 1977; U.S. Pat. No. 3,920,588, inventors Traeubel et al., issued Nov. 18, 1975; U.S. Pat. No. 3,679,540, inventors Zimmerman et al., issued Jul. 25, 1972; U.S. Pat. No. 3,351,495, inventors Larsen et al., issued Nov. 7, 1967; U.S. Patent Application Publication No. US 2015/0228948 A1, inventors Maruyama et al., published Aug. 13, 2015; U.S. Patent Application Publication No. US 2014/0147726 A1, inventor Toyoda, published May 29, 2014; U.S. Patent Application Publication No. US 2013/0280584 A1, inventor Matsumura, published Oct. 24, 2013; U.S. Patent Application Publication No. US 2013/0052735 A1, inventors DeRosa et al., published Feb. 28, 2013; U.S. Patent Application Publication No. US 2006/0051530 A1, inventors Schwarz et al., published Mar. 9, 2006; U.S. Patent Application Publication No. US 2003/0124324 A1, inventors Langley et al., published Jul. 3, 2003; PCT International Publication No. WO 2013/065738 A2, published May 10, 2013; European Patent Application Publication No. EP 2 881 163 A1, published Jun. 10, 2015; European Patent No. EP 1 157 653 B1, published Jan. 26, 2011; Chinese Patent Application No. CN 103213364 A, published Jul. 24, 2013; Chinese Patent Application No. CN 101541534 A, published Sep. 23, 2009; and Baldwin, “A Review of State-of-the-Art Separator Materials for Advanced Lithium-Based Batteries for Future Aerospace Missions,” NASA/TM-2009-215590 (2009).